Why would I need to create an additional user account?
If you have an accounts department that needs to receive invoices you can set them as a user just to receive invoices, or as a user to be able to view and pay invoices.
Maybe you have a web developer or an internal IT department that needs to log in and raise support tickets and/or update your domains etc. In this case you would create a user account for them and set the necessary permissions.
How do I do it?
It is very important to grant new users only the permissions that are required for them to perform their function and also to regularly maintain your user list and remove any permissions or users as required.